Histamine secretion from rat mast cells is accompanied by the oxidation of endogenous ascorbic acid to the free radical species. Furthermore, although their existence has been shown biochemically, free radicals could not be trapped during histamine secretion. The generation of oxidized ascorbic acid during secretion, however, suggests that this anti-oxidant may function as a radical scavenger, thereby preventing the trapping of any generated radicals. It is postulated that the relatively high level of ascorbic acid in mast cells serves to protect the cell membrane from oxidative damage. Ascorbic acid may, therefore, contribute to the survival of mast cells after an extensive secretory response.